1. Field of the Invention
This invention relates to the electrodeposition of zinc from an acid plating bath. Particularly, it relates to the plating of substrates which are then coated with a lacquer. The bath is formulated to eliminate blush which normally occurs when the plated substrate is subjected to bending stress or to elevated temperatures encountered for instance, when the lacquer is baked.
2. Discussion of the Prior Art
There are a number of commercially available acid zinc electroplating baths such as zinc sulfate, zinc chloride, zinc fluoroborate and pyrophosphate. The zinc plating bath typically includes a buffer such as an ammonium salt, one or more ductilizers, along with brighteners and other additives to improve throwing power and covering power. Suitable surfactants have included compounds such as polyalkylated napthols (U.S. Pat. No. 4,146,441 and U.S. Pat. No. 4,229,268); and alkoxylated alkyl phenols (U.S. Pat. No. 4,226,682, U.S. Pat. No. 4,070,256, U.S. Pat. No. 3,787,297).
A specialized application for acid zinc baths is the plating of shelves and other steel parts used inside of refrigerators. For appearance, the zinc layer must be bright and shiny. After plating, the parts normally are chromated and then are coated with a protective layer of a lacquer which is then baked by heating to an elevated temperature. Often this baking causes the zinc layer to become dull and hazy, a trait that is referred to as blush. The appearance of blush on the plated parts is a basis for the customer to reject the parts as unsatisfactory.
Prior attempts to correct the blush problem have not been successful. Repeated attempts at isolating the problem led to the conclusion that the lacquer was not the cause of the problem but that the surfactant in the plating bath was a major contributor to the thermally induced haze.
Often a part that undergoes flexing or bending after zinc plating likewise develops a haze and becomes dull and unattractive. Again, the problem appears to be related to the type and nature of surfactant used in the plating bath.
The development of a haze is particularly noticeable on parts that are plated at current densities above 14 or 15 amps per square foot, using steady or interrupted current. The haze is relatively independent of the nature or type of agitation, as well as the pH and the temperature of the bath.
The blush problem has been noted in a number of commercially available acid zinc plating baths, notably those that use ammonium or potassium salt as the source of zinc ions.